vhost-net: reduce one userspace copy when building XDP buff

vhost-net: reduce one userspace copy when building XDP buff

We used to do twice copy_from_iter() to copy virtio-net and packet
separately. This introduce overheads for userspace access hardening as
well as SMAP (for x86 it's stac/clac). So this patch tries to use one
copy_from_iter() to copy them once and move the virtio-net header
afterwards to reduce overheads.

Testpmd + vhost_net shows 10% improvement from 5.45Mpps to 6.0Mpps.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://patch.msgid.link/20250701010352.74515-2-jasowang@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>